The NPD Group today reported that iTunes is “proving to be a formidable competitor against free peer-to-peer (P2P) file-sharing services.” According NPD’s MusicWatch Digital service, iTunes’s tied with LimeWire as the second-most-popular digital music service in March, 2005. Both iTunes and LimeWire were used by 1.7 million households. The most popular digital music service that month was WinMX, which was used by 2.1 million households to download music. Paid a-la-carte music offerings from Napster and Real Networks also placed in the top ten, alongside other P2P services like iMesh and Kazaa, notes the report. Editor’s Note: NPD MusicWatch Digital information is collected continuously from the Windows PCs of 40,000 online panelists balanced to represent the online population of PC households.